Coffin’s averaging instrument or planimeter is shown in [Fig. 3373]. The diagram is traced by the point o, and the register wheel gives the area of the diagram.
Fig. 3374.
A quick method of approximating the mean effective pressure (or M. E. P. as it is called) of a diagram is to draw a line a b, in [Fig. 3374], touching the expansion curve at a, and so inclined that the space e is, as near as the eye can judge, equal to the space d. Then the line f drawn in the middle of the diagram, and measured on the scale of the spring that was used to take the diagram, represents the mean effective pressure, or M. E. P. of the diagram.
CALCULATING THE AMOUNT OF STEAM OR WATER USED.
The amount of water evaporated in the boiler is not accounted for by an indicator diagram or card, and the full reasons for this are not known.
It is obvious, however, that the loss, from the steam being unduly wet or containing water held in suspension, is not shown by the diagram, and this amount of loss will vary with the conditions.
Thus the loss from this cause will be less in proportion as the point of cut off occurs earlier in the stroke, because, as the water is at the same temperature as the steam, it will, as the temperature of the steam reduces from the expansion, evaporate more during the expansion period, doing so to a greater extent in proportion as the cut off is early, on account of there being a wider variation between the temperature of the steam at the point of cut off and at the end of the stroke. On the other hand, however, in proportion as the cut off is earlier, the proportionate loss from condensation during the live steam period is greater, because a greater length of the cylinder bore is cooled during the expansion period, and it has more time to cool in.
Whatever steam is saved by the compression, from the exhaust, must be credited to the engine in calculating the water consumption from the indicator card or diagram, since it fills, or partly fills, the clearance space.